Aquatint: A process for enhancing the tonal range of intaglio plates. A random etched pattern was produced on the plates by applying resin particles to the plates before etching. It was called a "ground", and was used as early as 1804.

Asphaltum: Synonyms bitumen, pitch, tar. Used by J.N. Niepce in 1826 for the oldest surviving photograph, and as an etch resist in various photolithographic processes. It was usually obtained from Trinidad or the Dead Sea (hence "Bitumen of Judea"). Pieces broken at temperatures below the softening range exhibit conchoidal or brittle fracture patterns, unlike tar from most other sources such as petroleum.

Baryta: Barium sulfate, a natural or synthesized mineral used as a white pigment; in photography, used as a paper coating under emulsions to hide paper texture.

Base: This is the bottom supporting material for photographs. It is one of the attributes listed in Section 1. The light-sensitive material may be coated directly on the base, as in salt prints; it may be in an emulsion layer on the base, or there may be a baryta layer between the base and emulsion.

Bichromate: The modern spelling is dichromate. The sensitizer for gum or gelatin processes such as carbon, carbro, collotype. Sodium, potassium, or ammonium dichromate have been used, for example K2Cr2O7.

Catalysis: Acceleration of the rate of a chemical reaction by a substance that does not become a constituent of the final reaction products. At one time it was thought to explain the appearance of the visible image in a printing-out paper, hence the name "catalysotype" in 1844.

Collodion: A solution of gun-cotton in ether and alcohol; gun- cotton is cotton reacted with nitric acid. It is highly flammable in liquid form. Towler [108] has a complete description. See "Guncotton" below.

Colloid: A suspension of particles in a liquid medium that fails to settle out. Examples are gelatin and gum arabic. Colloidal particles are of the order of 1000 times the size of the molecules of the supporting medium, making them visible under light microscopy.

DOP, or Developing-Out-Paper
: Photographic paper on which the visible image is chemically developed from an invisible latent image; first used about 1873, now the predominant type.

Embedded image
: The light-sensitive material is soaked into the paper rather than carried in a binder such as gelatin, collodion, or albumen. Paper fibers are easily visible in all parts of the matte image. Examples are salt prints and platinotypes.

Gelatin: Animal derivative first successfully used by Maddox in 1871 as a binder for silver bromide. It was also used as a safety film base in stripping films. The old spelling was "gelatine".

Grain: Visible development centers in a photographic image, not to be confused with paper fiber texture in salt prints, or the screen pattern in halftone engravings.

Ground: Roughening applied to intaglio plates to aid in retention of ink. The aquatint process was an example.

Gum Arabic or simply gum: A colloid produced from the bark of certain trees, used in the gum bichromate process.

Guncotton: The product of the reaction between certain organic substances such as cotton, and acids such as nitric or sulfuric. Guncotton is highly inflammable or explosive, and is soluble in ether and alcohol, yielding collodion, which has played an important role in photography as an emulsion base. Eder [48, 342-347] has a detailed discussion of the chemistry.

Halftone: The complete tonal range from white to black. A term often applied to inked prints made by various screen processes.

Halide: Chemical compounds containing the halogens fluorine, chlorine, bromine, and iodine. Silver halides have been the most important photographic compounds since 1839.

Index of Refraction
: A measure of the bending of light as it passes from one transparent medium to another, where the velocity of light differs.

Intaglio: ink printing process in which the ink is held in en- graved recesses below the main surface of the printing plate, as contrasted to relief printing where raised surfaces are inked, such as rubber stamps.

Lithograph: a paper print made by oil based inks transferred from an engraved master on stone.

Latent Image
: the invisible chemical change produced by light in a photosensitive material.

Matte: a surface from which reflected light is scattered in all directions; rougher than a glossy or smooth surface. Salt prints and platinotypes have matte surfaces because the paper is un- coated. Matte or semi-matte surfaces were produced on coated papers by the addition of starch, or by mechanical embossing or roughening.

Orthochromatic: A photosensitive surface sensitive to all colors of the visible spectrum except red; sometimes called 'color blind'.

Panchromatic: A photosensitive surface sensitive to all colors of the visible spectrum.

pH: a measure of acidity or alkalinity of a water solution, on a logarithmic scale of 0 to 14. Neutrality is 7.0 on this scale; above 7.0 represents alkaline solutions, while below 7.0 are acids. Alkaline solutions etch most glasses. "Buffered" paper contains alkaline materials such as calcium carbonate to neutralize acids that deteriorate paper. The pH scale is based on hydrogen ion concentration, and is meaningful only in water solutions; the pH of dry paper must be measured by certain archival procedures.

Plasticizer: an oil-like chemical added to polymers ("plastics") to make them soft or flexible.

POP or Printing-Out-Paper
: photographic paper on which an image appears spontaneously after light exposure without chemical development. Excess silver nitrate in the older emulsions often caused such photolytic development. Examples are albumen paper and some silver chloride and bromide papers; still used as proof paper for portraiture.

PPM: Parts Per Million, a measure of concentration, either by weight or by volume. Example: 0.1% = 1000 PPM.

Provenance: documentation on the known history of an artifact.

Resin: (1) Natural organic solids secreted from plants; example - rosin from pine trees. (2) Synthetic organic polymers used as "plastics".

Reticulation: a microscopic worm-like pattern in gelatin emulsions resulting from rapid and extreme temperature changes in solution, or drastic acid-alkaline cycling. It is a damage condition that is sometimes used for special effects. It was deliberately used in the collotype process to produce a random screen for halftone printing.

Sizing: a treatment applied to paper to produce a smooth base for subsequent coatings, to improve wet strength, and to reduce absorption of chemicals into the paper fibers. Many materials have been used, such as animal glue, tapioca, arrowroot, and gelatin, as well as modern resins.

Specular: Reflection of a coherent image from a smooth surface such as a mirror, as opposed to diffuse light from a matte surface. The direction of reflection is determined by the direction of the incident light, which can only occur when the height of irregularities does not exceed a small fraction of the wavelength of light.

Thermoplastic: a polymer whose solid shape can be reversibly altered by the action of heat and pressure. Examples are poly- vinyl chloride ("vinyl"), and polymethyl methacrylate (acrylic).

Thermosetting plastic: a polymer whose shape cannot be altered by the action of heat and pressure without the occurrence of decomposition. Examples are epoxies, and phenolic resins such as Bakelite.

Translucent: An optical property that passes diffuse light but not clear images.